Are you on Facebook? There are a lot of "Buy/Sell/Trade" groups in many areas. You can post ISO ("in search of") queries and if folks have it to sell, they'll respond, either with goods themselves or suggestions on where to find whatever it is you want.

And +1 on Craigslist and Freecycle, though both sites vary widely depending on where you are.

I live in an interesting area for secondhand. It seems the good stuff goes to consignment shops that charge as much as I can find it for on sale for at the end of the season.

For Goodwill and similar, I think you have to work there or go every day to catch stuff that's reasonably fashionable or even in good shape. There is huge demand due to the transience and other factors of this area.

I just learned from my daughter that there's one of these online swap groups in her neighborhood, but by invitation only - hmmm.

Depending on what household items are needed, maybe the Habitat for Humanity ReStores are another source:

http://www.habitat.org/restoresHabitat for Humanity ReStores are nonprofit home improvement stores and donation centers that sell new and gently used furniture, home accessories, building materials, and appliances to the public at a fraction of the retail price...

You can also register your email address on "estatesales.net" to get local estatesales each week - many include pictures of what type of items they're selling and it's not just "expensive" homes that do estate sales.